Issue in Updating View

Question

Hi All,
I have created a view as below :
Create or replace view test as
select employee_id,first_name,last_name,department_id,department_name from
(select employee_id,first_name,last_name,d.department_id department_id,d.department_name department_name
from employees e,departments d
where e.department_id = d.department_id
and d.department_id = 10)
View created with the following record :
EMPLOYEE_ID FIRST_NAME LAST_NAME DEPARTMENT_ID DEPARTMENT_NAME
200 Jennifer Whalen 10 Administration
Employee_id is the pk of employees table and department_id is the pk of the Department table. Also my view contains no duplicate value from any of the tables.
As per my understanding my View is a Key-Preserved View. But i am not able to update the view as below :
update test set department_name = 'Admin'
where department_id = 10
When i am trying to update the view i am getting the error as
update test set department_name = 'Admin'
ERROR at line 1:
ORA-01779: cannot modify a column which maps to a non key-preserved table
Can anybody please tell me what is wrong in it?

Answer 1

here is an extract from the Oracle documentation,
-It is not necessary that the key or keys of a table be selected for it to be key preserved. It is sufficient that if the key or keys were selected, then they would also be key(s) of the result of the join.
when u check your view that has been created, employee_id preserves the key property but the department table columns does not.
Further it also explains that,
-The key-preserving property of a table does not depend on the actual data in the table. It is, rather, a property of its schema and not of the data in the table. For example, if in the EMP_TAB table there was at most one employee in each department, then DEPT_TAB.DEPTNO would be unique in the result of a join of EMP_TAB and DEPT_TAB, but DEPT_TAB would still not be a key-preserved table.
hope this helps you